[Patch 4.14 2/4] net: sched: crash on blocks with goto chain action

[Cong Wang <xiyou.wangcong@gmail.com>]

From: Roman Kapl <code@rkapl.cz>

[ Upstream commit a60b3f515d30d0fe8537c64671926879a3548103 ]

tcf_block_put_ext has assumed that all filters (and thus their goto
actions) are destroyed in RCU callback and thus can not race with our
list iteration. However, that is not true during netns cleanup (see
tcf_exts_get_net comment).

Prevent the user after free by holding all chains (except 0, that one is
already held). foreach_safe is not enough in this case.

To reproduce, run the following in a netns and then delete the ns:
    ip link add dtest type dummy
    tc qdisc add dev dtest ingress
    tc filter add dev dtest chain 1 parent ffff: handle 1 prio 1 flower action goto chain 2

Fixes: 822e86d997 ("net_sched: remove tcf_block_put_deferred()")
Signed-off-by: Roman Kapl <code@rkapl.cz>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 net/sched/cls_api.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index 1451a56a8f93..ddae7053b745 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -282,7 +282,8 @@ static void tcf_block_put_final(struct work_struct *work)
 	struct tcf_chain *chain, *tmp;
 
 	rtnl_lock();
-	/* Only chain 0 should be still here. */
+
+	/* At this point, all the chains should have refcnt == 1. */
 	list_for_each_entry_safe(chain, tmp, &block->chain_list, list)
 		tcf_chain_put(chain);
 	rtnl_unlock();
@@ -290,17 +291,23 @@ static void tcf_block_put_final(struct work_struct *work)
 }
 
 /* XXX: Standalone actions are not allowed to jump to any chain, and bound
- * actions should be all removed after flushing. However, filters are now
- * destroyed in tc filter workqueue with RTNL lock, they can not race here.
+ * actions should be all removed after flushing.
  */
 void tcf_block_put(struct tcf_block *block)
 {
-	struct tcf_chain *chain, *tmp;
+	struct tcf_chain *chain;
 
 	if (!block)
 		return;
 
-	list_for_each_entry_safe(chain, tmp, &block->chain_list, list)
+	/* Hold a refcnt for all chains, except 0, so that they don't disappear
+	 * while we are iterating.
+	 */
+	list_for_each_entry(chain, &block->chain_list, list)
+		if (chain->index)
+			tcf_chain_hold(chain);
+
+	list_for_each_entry(chain, &block->chain_list, list)
 		tcf_chain_flush(chain);
 
 	INIT_WORK(&block->work, tcf_block_put_final);
-- 
2.13.0